1. Field of the Invention
The present invention relates to turbo-molecular pumps used in semiconductor manufacturing apparatus, an electronic microscope, a surface analysis apparatus, a mass spectrograph, a particle accelerator, a nuclear fusion experiment apparatus, and so forth, and more particularly, the present invention relates to a turbo-molecular pump in which its connecting portion with a vacuum chamber is improved.
2. Description of the Related Art
In a process such as dry etching, chemical vapor deposition (CVD), or the like performed in a high-vacuum process chamber in semiconductor manufacturing step, a vacuum pump such as a turbo-molecular pump is used for producing a high vacuum in the process chamber by exhausting gas from the process chamber, as shown in, for example, Japanese Unexamined Patent Application Publication No. 20001-291586.
FIG. 12 illustrates a conventional turbo-molecular pump used for the above purposes. The turbo-molecular pump shown in FIG. 12 is a composite pump having a turbo-molecular pump unit and a groove pump unit.
As shown in FIG. 12, the turbo-molecular pump has a rotor 42 having a plurality of rotor blades 41 and a rotor shaft 43 integrally fixed to the rotor 42 along the rotation center axis thereof, both being housed in a pump case 1, so as to form a high-speed rotating body. The rotor shaft 43 is rotatable supported by upper and lower magnetic bearings 46 which are disposed between the rotor shaft 43 and a stator column 45 disposed so as to be erected at the lower part of a pump base 44 for supporting the pump case 1 and also by a magnetic bearing 46S which is disposed between the pump base 44 and the rotor shaft 43. The high-speed rotating body rotates at a high speed of about 400 m/s with respect to the peripheral velocity of the rotor blades, driven by a drive motor 47 which is incorporated between the upper and lower magnetic bearings 46 and between the stator column 45 and the rotor shaft 43.
While rotating at such a high speed, by inhaling gas from a gas suction port 48 disposed above the rotor 42 and then by exhausting it from a gas vent 49 disposed below the rotor 42, the turbo-molecular pump produces a high vacuum in a vacuum chamber 3 connected to the gas suction port 48 with flanges 2 and 4 in a semiconductor manufacturing process or the like.
The above-mentioned evacuating operation is performed by a turbo-molecular pump mechanism portion A and a groove pump mechanism portion B, that is, upper and lower parts of the turbo-molecular pump, respectively.
More particularly, the turbo-molecular pump mechanism portion A is formed by the plurality of rotor blades 41 and a plurality of stator blades 50 fixed to the pump case 1 such that the rotor blades 41 and the stator blades 50 are alternately disposed. With this structure, gas molecules from the gas suction port 48 in a high vacuum is sent downwards in the figure by the interaction between the high-speed rotating rotor blades 41 and the stationary stator blades 50 so as to perform an exhausting operation.
The groove pump mechanism B is formed by a rotating cylindrical surface 42b, that is, the outer peripheral surface of a skirt portion 42a serving as a lower half of the rotor 42 and by a threaded stator 51 fixed in the pump case 1 so as to closely surround the rotating cylindrical surface 42b. With this structure, the gas molecules sent from the turbo-molecular pump mechanism portion A to spiral thread grooves 52 carved on the inner surface of the threaded stator 51 is sent into the gas exhaust port 49 along the thread grooves 52 by the rotating cylindrical surface 42b of the skirt portion 42a of the rotor 42 rotating at high speed so as to perform an exhausting operation of the gas in a relatively low degree of vacuum.
The rotor blades 41, the rotor 42, the stator blades 50, the chamber 3 connected to the gas suction port 48, and the like are usually composed of a light alloy, especially an aluminum alloy among others since the aluminum alloy has good machinability and is thus easily and precisely processed. Meanwhile, the aluminum alloy has a relatively small strength and sometimes causes a creep fracture depending on its use conditions.
Among the above-mentioned components, the rotor blades 41 and the rotor 42 integrally formed with the rotor blades 41 undergo a dynamic balancing operation during their assembling process in order to withstand a high-speed rotation. The dynamic balancing operation is usually performed by carving a small amount out of the upper and lower surfaces of the rotor 42 with a drill or the like. When the dynamical balance of the rotating body is well achieved, the high-speed rotating body can rotate at high speed and thus the pump can operate quietly with little vibration. However, during high-speed rotation, a centrifugal force causes stress concentrations to occur around fine drilled bores formed for dynamic balance on the upper and lower surfaces of the rotor 42, and also, when a process gas causes the upper and lower surfaces to corrode around some of the drilled bores, cracks occur around the corroded portions of these surfaces. Thus, both problems may cause a brittle fracture of the high-speed rotating body.
This problem is not limited to the drilled bores formed for dynamic balance. When some kind of defect exists even in other parts of the high-speed rotating body, a stress concentration occurs at the defect, thereby causing a brittle fracture of the high-speed rotating body.
Since the breakage of the rotor 42 starting at one of the stress concentration points thereof occurs when the rotor 42 and the rotor blades 41 are rotating at high speed, its breaking energy is so large that the breaking energy quickly has an impact on and accordingly breaks the entire rotor 42 and rotor blades 41, and thus broken pieces of these components are caused to fly out due to a centrifugal force and forcefully stop rotation of the drive motor 47 to rotate. A reaction of the forceful stop causes the motor casing (stator column) 45 to receive a large torque (hereinafter, referred to as a damaging torque) and thus pump-chamber fastening bolts 6 for fastening the pump to the vacuum chamber 3 to be broken. As a result, the fall of the pump may lead to break a part of the semiconductor production equipment or to a serious accident causing injury or death.
Vacuum pumps having a large capacity have been increasingly used in recent years. As the vacuum pump becomes larger, the damaging torque due to a centrifugal force becomes larger, thereby resulting in a larger risk of a falling accident of the pump.
In order to prevent the fall of the pump by limiting the above-mentioned breakage so as to be small within the pump, various improvements for preventing the pump-chamber fastening bolts from being broken even when the damaging torque occurs have been heretofore attempted.
Unfortunately, these improvements have not assured that the pump-chamber fastening bolts have no risk of being broken at all.